1. Field of the Invention
The present invention relates generally to ball joints which are used in steering apparatuses or suspensions for vehicles and, more particularly, to a ball joint which is manufactured by insert molding.
2. Description of the Related Art
Generally, ball joints which are made only of metal and are manufactured in an assembly manner are widely used. Recently, insert-molded ball joints having housings made of synthetic resin have been proposed.
As techniques relevant to the present invention, representative examples of an insert-molded ball joint were proposed by the applicant of the present invention in Korean Patent Registration No. 10-0858689 (date: Sep. 9, 2008) which was entitled “Insert-molded ball joint having grease grooves and method of manufacturing the same” and in Korean Patent Registration No. 10-0880844 (date: Jan. 21, 2009) which was entitled “Insert-molded ball joint and method of manufacturing the same”.
In the technique of No. 10-0858689, a ball stud is inserted into a mold and a housing is formed by injection molding, so that the housing and the ball stud come into direct contact with each other. In the technique of No. 10-0880844, after a spherical head of a ball stud is assembled with a ball seat, the ball stud and the ball seat are inserted into a mold and then a housing is formed by injection molding, so that the ball stud is separated from the housing by the ball seat.
The technique of No. 10-0858689 has an advantage of a simple manufacturing process but is problematic in that because the housing is formed on the ball stud by direct injection molding, rotational frictional torque of the ball stud with respect to the housing is relatively large.
On the other hand, in the technique of No. 10-0880844, because the ball stud rotates with respect to the ball seat, rotational friction torque of the ball stud is markedly reduced.
However, in the case of the ball joint of No. 10-0880844, resistance strength against a removal of the ball stud from the housing, in other words, pull-out strength, is relatively small. Furthermore, because the entirety of the spherical head of the ball stud is installed in the housing by the ball seat, there is a problem in that play of the ball stud is increased.